Cooling agents for cooling systems in fuel cell drives

ABSTRACT

The invention relates to antifreeze concentrates for cooling systems in fuel cell drives, from which are produced ready-to-use aqueous cooling agent compositions having a maximum conductivity of 50 μm/cm, based on alkylene glycols or the derivatives thereof, and containing orthosilicic acid esters of formula (I) wherein the variables R 1  to R 4  are the same or different and represent C 1 -C 20  alkyl substituents, C 2 -C 20  alkenyl substituents, C 1 -C 20  hydroxyalkyl substituents, optionally substituted C 6 -C 12  aryl substituents and/or glycol ether substituents of formula (CH 2 —CH 2 —O) a —R 5  wherein R 5  represents hydrogen or C 1 -C 5  alkyl and n represents a number between 1 and 5.

[0001] The present invention relates to coolants for cooling systems infuel cell drives, in particular for motor vehicles, based on alkyleneglycols or derivatives thereof, which contain orthosilicic esters ascorrosion inhibitors.

[0002] Fuel cells for mobile use in motor vehicles must be capable ofbeing operated also at low outdoor temperatures down to about −40° C.; acoolant circulation protected from freezing is therefore essential.

[0003] The use of radiator antifreezes conventionally used in internalcombustion engines would not be possible in the case of fuel cellswithout complete electrical insulation of the cooling ducts, since,owing to the salts contained therein as corrosion inhibitors, theseantifreezes have too high an electrical conductivity, which wouldadversely affect the operation of the fuel cell.,

[0004] DE-A 198 02 490 (1) describes fuel cells having a coolingcirculation which contains an antifreeze and in which a paraffinicisomer mixture having a pour point of less than −40° C. is used as acoolant. However, the flammability of such a coolant is disadvantageous.

[0005] EP-A 1 009 050 (2) discloses a fuel cell system for automobiles,in which air is used as a cooling medium. The disadvantage here,however, is that air is known to be a poorer heat conductor than aliquid cooling medium.

[0006] WO 00/17951 (3) describes a cooling system for fuel cells, inwhich a pure monoethylene glycol/water mixture in the ratio 1:1, withoutadditives, is used as a coolant. Since, owing to the absence ofcorrosion inhibitors, there has been no corrosion protection at allagainst the metals present in the cooling system, the coolingcirculation contains an ion exchange unit to maintain the purity of thecoolant and to ensure a low specific conductivity for a longer time,with the result that short-circuits and corrosion are prevented. Anionicresins, for example of the strongly alkaline hydroxyl type, and cationicresins, for example those based on sulfo groups, are mentioned assuitable ion exchangers, and oth r filtration units, for example activecarbon filters, are mentioned.

[0007] The structure and the mode of operation of a fuel cell forautomobiles, in particular a fuel cell comprising an electron-conductingelectrolyte membrane (PEM fuel cell, polymer electrolyte membrane fuelcell) are described by way of example in (3), aluminum being a preferredmetal component in the cooling circulation (radiator).

[0008] The use of silicon compounds, generally in the form of silicates,as corrosion inhibitors in radiator antifreezes for conventionalinternal combustion engines operated using gasoline or diesel fuel haslong been known, for example from: G. Reinhard, “AktiverKorrosionsschutz in wäβrigen Medien”, pages 87-98, expert-Verlag 1995(ISBN 3-8169-1265-6).

[0009] EP-A 105 803 (4) discloses the use of orthosilicic esters inaddition to ionic corrosion inhibitors in radiator antifreezes forautomobiles having conventional gasoline or diesel internal combustionengines.

[0010] The use of orthosilicic esters as corrosion inhibitors incoolants for cooling systems in fuel cell drives is unknown to date.

[0011] A principal problem in the case of cooling systems in fuel celldrives is the maintenance of low electrical conductivity of the coolantin order to ensure safe and trouble-free operation of the fuel cell andpermanently to prevent short-circuits and corrosion.

[0012] Surprisingly, it has now been found that the duration of lowelectrical conductivity in a cooling system based on alkyleneglycol/water, in particular if, according to (3), it contains anintegrated ion exchanger, can be substantially increased by adding smallamounts of orthosilicic esters; in practice, this has the advantage thatthe time intervals between two coolant changes in the case of fuel celldrives can be further extended, which is of interest particularly in theautomotive sector.

[0013] Accordingly, we have found antifreeze concentrates for coolingsystems in fuel cell drives, from which ready-to-use aqueous coolantcompositions having a conductivity of not more than 50 μS/cm result andwhich are based on alkylene glycols or derivatives thereof, whichconcentrates contain orthosilicic esters of the formula (I)

[0014] where R¹ to R⁴ are identical or different and are C₁- toC₂₀-alkyl, C₂- to C₂₀-alkenyl, C₁- to C₂₀-hydroxyalkyl, unsubstituted orsubstituted C₆- to C₁₂-aryl and/or a glycol ether substituent of theformula —(CH₂—CH₂—O)_(n)—R⁵, where R⁵ is hydrogen or C₁— to C₅-alkyl andn is from 1 to 5.

[0015] Preferred antifreeze concentrates here are those from whichready-to-use aqueous coolant compositions having a silicon content offrom 2 to 2000, in particular from 10 to 1000, preferably from 25 to500, especially from 40 to 250, ppm by weight, from orthosilicic estersof the formula I, result.

[0016] Typical examples of orthosilicic esters (I) used according to theinvention are pure tetraalkoxysilanes, such as tetramethoxysilane,tetraethoxysilane, tetra-n-propoxysilane, tetraisopropoxysilane,tetra-n-butoxysilane, tetra-tert-butoxysilane,tetra(2-ethylbutoxy)silane and tetra(2-ethylhexyloxy)silane, andfurthermore tetraphenoxysilane, tetra(2-methylphenoxy)silane,tetravinyloxysilane, tetraallyloxysilane, tetra(2-hydroxyethoxy)silane,tetra(2-ethoxyethoxy)silane, tetra(2-butoxyethoxy)silane,tetra(1-methoxy-2-propoxy)silane, tetra(2-methoxyethoxy)silane andtetra[2-[2-(2-methoxyethoxy)ethoxy]ethoxy]silane. The orthosilicicesters (I) used preferably have four identical variables R¹ to R⁴.

[0017] In a preferred embodiment, orthosilicic esters (I) in which R¹ toR⁴ are identical and are C₁- to C₄-alkyl or a glycol ether substituentof the formula —(CH₂—CH₂—O)_(n)—R⁵, where R⁵ is hydrogen, methyl orethyl and n is 1, 2 or 3, are used.

[0018] Said orthosilicic esters (I) are either commercially available orcan be prepared by simple transesterification of one equivalent oftetramethoxysilane with four equivalents of the correspondinglonger-chain alcohol or phenol and by distilling off methanol.

[0019] Ready-to-use aqueous coolant compositions which have aconductivity of not more than 50 μS/cm and substantially comprise

[0020] (a) from 10 to 90% by weight of alkylene glycols or derivativesthereof,

[0021] (b) from 90 to 10% by weight of water and

[0022] (c) from 2 to 2000, preferably from 25 to 500, ppm by weight ofsilicon from orthosilicic esters of the formula I can be prepared fromthe antifreeze concentrates by dilution with ion-free water. The sum ofall components is 100% by weight here.

[0023] The present invention thus also relates to ready-to-use aqueouscoolant compositions for cooling systems in fuel cell drives, whichsubstantially comprise

[0024] (a) from 10 to 90% by weight of alkylene glycols or derivativesthereof,

[0025] (b) from 90 to 10% by weight of water and

[0026] (c) from 2 to 2000, preferably from 25 to 500, ppm by weight ofsilicon from orthosilicic esters of the formula I

[0027] and which are obtainable by diluting said antifreeze concentrateswith ion-free water. The sum of all components is 100% by weight here.

[0028] The novel ready-to-use aqueous coolant compositions have aninitial electrical conductivity of not more than 50, in particular 25,preferably 10, especially 5, RS/cm. The conductivity is kept at this lowlevel in continuous operation of the fuel cell drive over several weeksor months, particularly if a cooling system comprising integrated ionexchanger-is-used in the fuel cell drive.

[0029] The pH of the novel ready-to-use aqueous coolant compositionsdecreases over the operating time-substantially more slowly than in thecase of cooling liquids not containing added orthosilicic esters. The pHis usually from 4.5 to 7 in the case of fresh coolant compositionsaccording to the invention and can decrease to 3.5 in continuousoperation.

[0030] The ion-free water used for dilution may be pure distilled orbidistilled water or, for example, water demineralized by ion exchange.

[0031] The preferr d weight ratio in which an alkylene glycol or aderivative thereof is mixed with water in the ready-to-use aqueouscoolant compositions is from 25:75 to 80:20, in particular from 35:65 to75:25, preferably from 50:50 to 70:30, especially from 55:45 to 65:35.In particular, monoethylene glycol, but also monopropylene glycol,polyglycols, glycol ethers or glycerol, in each case alone or asmixtures thereof, can be used as alkylene glycol components orderivatives thereof. Monoethylene glycol alone or mixtures containingmonoethylene glycol as the main component, i.e. having a content of morethan 50, in particular more than 80, especially more than 95, % byweight in the mixture, with other alkylene glycols or derivatives ofalkylene glycols are particularly preferred.

[0032] The dosage of the respective orthosilicic esters (I) in theready-to-use aqueous coolant compositions is calculated from the abovedata by means of the silicon content based on (I).

[0033] The novel antifreeze concentrates themselves, from which thedescribed ready-to-use aqueous coolant compositions result, can beprepared by dissolving the orthosilicic esters (I) in alkylene glycolsor derivatives thereof, which may be used in anhydrous form or with alow water content (up to about 10, in particular up to 5, % by weight).

[0034] The present invention also relates to the use of orthosilicicesters of the formula I

[0035] where R¹ to R⁴ are identical or different and are C₁- toC₂₀-alkyl, C₂- to C₂₀-alkenyl, C₁- to C₂₀-hydroxyalkyl, unsubstituted orsubstituted C₆- to C₁₂-aryl and/or a glycol ether substituent of theformula —(CH₂—CH₂—O)_(n)—R⁵, where R⁵ is hydrogen or C₁- to C₅-alkyl andn is from 1 to 5,

[0036] for the preparation of antifreeze concentrates for coolingsystems in fuel cell drives, in particular for motor vehicles, based onalkylene glycols or derivatives thereof.

[0037] The present invention furthermore relates to the use of theantifreeze concentrates described for the preparation of ready-to-useaqueous coolant compositions having a conductivity of not more than 50RS/cm for cooling systems in fuel cell drives, in particular for motorvehicles.

EXAMPLES

[0038] The examples which follow illustrate the invention withoutrestricting it.

[0039] The novel coolant compositions were tested with regard to theirsuitability for fuel cell drives by the test described below, incomparison with a coolant composition according to (3):

[0040] Description of Test:

[0041] Five aluminum test metals (vacuum-soldered Al, designation: EN-AW3005, solder-plated on one side with 10% by weight of EN-AW 4045;dimensions: 58×26×0.35 mm, having a hole of 7 mm diameter) were weighed,connected nonconductively by means of a plastics screw with nut andTeflon washer and placed on two Teflon supports in a 1 l beaker withground glass joint and glass cover. Thereafter, 1000 ml of test liquidwere introduced and a small fabric bag containing 2.5 g of an ionexchanger (mixed bed ion exchanger resin AMBERJET UP 6040 RESIN fromRohm+Haas) was suspended in the liquid. The beaker was closed air-tightwith the glass cover and heated to 88° C. and the liquid was stirredvigorously with a magnetic stirrer. The electrical conductivity wasmeasured at the beginning of the test and after 7 and 42 (or after 77)days (conductivity meter LF 530 from WTW/Weilheim). The test was thenterminated; the aluminum samples were assessed visually and, afterpickling with aqueous chromic acid/phosphoric acid, were evaluatedgravimetrically according to ASTM D 1384-94.

[0042] The results are shown in Table 1 below. TABLE Example 1: Example2: Comparative 60% by 60% by volume of example volume of monoethylene(according to monoethylene glycol WO 00/17951): glycol 40% by volume 60%by 40% by volume of water volume of of water 3600 ppm by weightmonoethylene 742 ppm by of tetra[2-[2- glycol 40% weight ofmethoxyethoxy) Coolant 40% by volume tetraethoxy- ethoxy] ethoxy]-composition: of water silane silane Electrical conductivity [μS/cm]Beginning of 2.0 0.8 2.6 test: After 7 days: 2.3 0.8 2.2 After 42 days:36.2 3.0 14.4 After 77 days: — — 18.6 pH: 6.9 6.6 4.7 Beginning of testEnd of test: 2.9 4.0 3.6 Appearance slightly stained stained stainedAluminum samples after the test: Weight change [mg/cm²] After pickling:1 −0.05 −0.02 −0.02 2 −0.04 −0.01 −0.02 3 −0.04 −0.02 −0.04 4 −0.04−0.02 −0.04 5 −0.03 −0.02 −0.04 Mean value of −0.04 −0.02 −0.03 the 5samples Solution after yellowish, colorless, yellowish, clear end oftest clear clear

[0043] In the mixture of monoethylene glycol and water, the volume ratioof 60:40 corresponds to a weight ratio of 62.5:37.5.

[0044] In the novel examples 1 and 2, the orthosilicic esters weremetered so that a silicon content of 100 ppm by weight in each case waspresent in the cooling liquid.

[0045] The results show that a very low electrical conductivity of lessthan 5 μS/cm was still present even after an uninterrupted test periodof 42 days in the case of the novel example 1, whereas a substantialdeterioration had occurred, with an increase to virtually 40 μS/m, inthe case of the additive-free coolant according to WO 00/17951 (3).However, with the novel example 2 which was slightly poorer comparedwith example 1 after 42 days, the specific conductivity was still about50% lower even after a test period of 72 days than in the case of thecomparative example after a test duration of 42 days.

[0046] In no case did significant corrosion occur on the aluminumsamples.

We claim:
 1. A method of protecting fuel cell driv s from corrosionusing cooling systems which are based on antifreeze concentrates basedon alkylene glycols or derivatives thereof, from which ready-to-useaqueous coolant compositions having a conductivity of not more than 50μS/cm result, wherein the antifreeze concentrates contain orthosilicicesters of the formula I

where R¹ to R⁴ are identical or different and are C₁- to C₂₀-alkyl, C₂-to C₂₀-alkenyl, C1- to C₂₀-hydroxyalkyl, unsubstituted or substitutedC₆— to C₁₂-aryl and/or a glycol ether substituent of the formula—(CH₂—CH₂—O)_(n)—R⁵, where R⁵ is hydrogen or C₁- to C₅-alkyl and n isfrom 1 to
 5. 2. A method as claimed in claim 1, wherein the antifreezeconcentrates result in ready-to-use aqueous coolant compositions havinga silicon content of from 2 to 2000, preferably from 25 to 500, ppm byweight, from orthosilicic esters of the formula I.
 3. A method asclaimed in claim 1 or 2, wherein the antifreeze concentrates containorthosilicic esters of the formula I, in which R¹ to R⁴ are identicaland are C₁- to C₄-alkyl or a glycol ether substituent of the formula—(CH₂—CH₂—O)_(n)—R⁵, where R⁵ is hydrogen, methyl or ethyl and n is 1, 2or
 3. 4. A method as claimed in any of claims 1 to 3, wherein thealkylene glycol used for the antifreeze concentrates is monoethyleneglycol.
 5. A method as claimed in any of claims 1 to 4, whereinready-to-use aqueous coolant compositions which have a conductivity ofnot more than 50 μS/cm and substantially comprise (a) from 10 to 90% byweight of alkylene glycols or derivatives thereof, (b) from 90 to 10% byweight of water and (c) from 2 to 2000, preferably from 25 to 500, ppmby weight of silicon from orthosilicic esters of the formula I areproduced from the antifreeze concentrates by dilution with ion-freewater.
 6. The use of an antifreeze concentrate as claimed in claim 1, 3or 4 for the preparation of ready-to-use aqueous coolant compositionshaving a conductivity of not more than 50 μS/cm for cooling systems infuel cell drives.